As is known, the Otto thermodynamic cycle involves adiabatic compression (i.e., the compression stroke) from some inlet condition of pressure and volume as the piston moves upwardly in the cylinder. This is followed by an isochoric (i.e., constant volume) pressure rise due to ignition, and then adiabatic expansion during the power stroke. The hot gases are then exhausted during the exhaust stroke. The Diesel cycle is similar except that adiabatic compression is followed first by an isochoric pressure rise due to ignition and then by an isobaric addition of heat (i.e., burning of fuel) prior to adiabatic expansion in the power stroke.
In both the Otto and Diesel cycles, the efficiency of the engine is: ##EQU1## where Q.sub.A is the heat added and Q.sub.R is the heat rejected, much of this rejected heat being in the exhaust gases. At a typical compression ratio of 8, the ideal efficiency of an engine operating on the Otto cycle is about 56.5%; but in actual practice, the efficiency is only about 30%. This is reflected in much higher exhaust temperatures than those which are calculated from ideal, theoretical calculations. For example, almost 60% of the energy supplied by combustion of fuel in an internal combustion engine is expelled with the exhaust gases. Needless to say, if some of this otherwise wasted energy can be fed back to the combustion cycle, considerable improvement in efficiency can be achieved.